A fingerprint sensor that captures fingerprint images from persons to be authenticated normally has a sensor surface that is greater than finger. In recent years, however, sensor surfaces are smaller than finger, so that fingerprint sensors can be mounted on small-sized information devices such as portable telephone devices and PDAs (Personal Digital Assistants). In such a case, partial images that are successively captured through a sensor surface are combined to form an image of an entire fingerprint.
Japanese Patent Application Publication Nos. 10-091769, 11-253428 and 2003-208620 disclose sweep-type fingerprint sensors as fingerprint sensors of the above type. In a sweep-type fingerprint sensor, there is friction between the finger skin and the sensor surface of the fingerprint sensor. Therefore, the finger is stuck on the sensor surface of the fingerprint sensor, and makes a relative movement when released.
Japanese Patent Application Publication Nos. 2003-271964, 2002-216116 and 2006-260359 disclose techniques for correcting image distortions caused by such movements. International Publication No. WO 2004/026139 discloses a technique for correcting partial images having distortions caused by delays in detections depending on the scanning method of the image capturing device, with the use of the relative positions of the partial images and the time information.
In those conventional techniques, changes caused in relative positions in a very short period of time are ignored. In cases where a discarding process is carried out on the side of the fingerprint sensor device, small errors are allowed so that the discarding is exhaustively performed, regardless of the influence of noise or the like. Here, the discarding process is a process of reducing the data amount by stopping output of some partial images among successive partial images.
However, there is a problem with the simple discarding process disclosed in conventional techniques. As described above, when there is friction between the skin of finger and the sensor surface of the fingerprint sensor, the finger is stuck on the sensor surface of the fingerprint sensor, and makes a relative movement when released. When such intermittent relative movements are made, the relative speed of the finger skin on the sensor surface varies in a very short period of time. As a result, some of the fingerprint information might be lost by the discarding. This situation will be hereinafter referred to as excess discarding.
When there is excess discarding, a region similar to some other region might not be detected from a region where partial images overlap with each other. In such a case, the frequency at which the person to be authenticated is rejected or a third party is accepted becomes higher. At the same time, due to the information loss caused by the excess discarding of partial images, there is the problem that changes caused in relative positions within a very short period of time cannot be detected with accuracy.
As described above, when there are rapid changes in a very short period of time, the distortion sizes vary in the partial image plane. In other words, due to the changes caused in relative position within a very short period of time, the distortions in the partial image plane become uneven. Where there are uneven distortions, regions similar to other regions are not detected, and the frequency at which the person to be authenticated is rejected or a third party is accepted becomes higher.